Squirrel cage rotors for modern induction motors typically include a core comprised of a stack of steel laminations and an aluminum squirrel cage conductor arrangement, usually formed as a die casting. Manufacturing techniques have been perfected to the point where these rotors are mass produced with a high probability of uniformity and high quality. There are, however, a number of possibilities for deficiencies, including porosity or impurities in the aluminum casting and open circuits in the squirrel cage conducting bars which can affect the electrical resistance of the rotor, poor insulation between the squirrel cage conductors and the iron core which can produce undesired variations in the effective skew, and various other manufacturing defects. Thus it is desirable to test dynamoelectric machine rotors economically and reliably to detect such defects.
Because quality problems are generally infrequent, it is not economical to perform expensive tests on every individual rotor. However, since hidden defects do occur, in order to maintain a high degree of quality control there is a need to perform low cost tests on each rotor before it is assembled with a stator to form a complete machine. Further, it can be desirable to obtain information on the resistance, reactance and effective skew of the rotors for evaluation of defects, manufacturing processes and quality control.
A number of prior art methods have been developed in an attempt to test squirrel cage rotors. Some, such as that disclosed in U.S. Pat. No. 2,844,794, assigned to the assignee of the present invention, require the use of the dynamoelectric machine stator core, while others use destructive testing techniques. One non-destructive prior art technique for testing rotors independent of the stator is disclosed in U.S. Pat. No. 3,861,025, assigned to the assignee of the present invention. This technique involves rotating the rotor in a static magnetic field and evaluating the waveform of the resulting induced voltages displayed on an oscilloscope. This technique requires extensive operator training to interpret the oscilloscope display, and has inherent limitations on the results that can be achieved. Another prior art testing technique utilizes a stator fixture excited by a fixed AC current into which the rotor is placed and manually rotated to obtain a peak power measurement (i.e. power into the rotor) using a pick-up coil. By using the current measurement, the impedance of the rotor can be obtained, but separate resistance, reactance and skew information can not be determined.
It is accordingly an object of the present invention to provide a novel and improved method and apparatus for non-destructive testing of dynamoelectric machine rotors.
It is another object of the invention to provide a novel, economical, and reliable method and apparatus for non-destructive measurement of the resistance and reactance of dynamoelectric machine rotors.
It is yet another object of the invention to provide a novel, economical and reliable method and apparatus for non-destructive measurement of the effective skew of dynamoelectric machine rotors.
It is yet another object of the invention to provide a novel, economical, and reliable method and apparatus for non-destructive testing of dynamoelectric machine rotors which provides automatic pass/fail determinations.
It is still another object of the invention to provide a novel, economical, and reliable method and apparatus for non-destructive testing of dynamoelectric machine rotors including the measurement of resistance, reactance and skew and a detailed statistical comparison and evaluation of the measurement results, as well as automatic identification of defective rotors.